The salt of dichromic acid, such as alkali metal dichromate, particularly sodium dichromate and potassium dichromate, is an important basic raw material in inorganic chemical engineering, and has been widely used in the fields of chemical engineering, metallurgy, pigments, tanning, dyeing and finishing, electroplating, medicine, etc.
For preparation of dichromate, there are mainly three production methods that have been industrialized. For example, in preparation of sodium dichromate, all of these methods employ sodium chromate solution as the starting material. In most countries, including China, a sulfuric-acid method is adopted, in which sodium chromate solution is acidified with sulfuric acid, as shown in 2Na2CrO4+H2SO4═Na2Cr2O7+Na2SO4, then concentrated by evaporation to precipitate sodium sulfate crystals that are then filtered out, and the filtrate is further concentrated by evaporation and then cooled to precipitate crystals, so as to afford the product sodium dichromate dihydrate. The chromate factories of Bayer AG (Germany) and of Corpus Christi, Tex., U.S. (both factories have been closed one after the other) used to produce sodium dichromate by an electrolysis method in which sodium chromate solution is fed into the anode chamber of an electrolytic tank having two chambers partitioned with an ion-exchange membrane, then electrolyzed into sodium dichromate under a direct current, and the liquid in the anode chamber is collected and evaporated to precipitate crystals to yield sodium dichromate dihydrate. Recently a similar method by the Qinghai Institute of Salt Lakes has also been published (see CN101481802A). A new chromate factory, built by Bayer AG (Germany) in Newcastle, South Africa, adopts a carbonation method in which sodium chromate solution is carbonated with pressurized carbon dioxide in a carbonation column, as shown in 2Na2CrO4+2CO2+H2O═Na2Cr2O7+2NaHCO3, most sodium bicarbonate is precipitated as crystals because of its low solubility and then filtered out at a decreased temperature with the partial pressure of carbon dioxide maintained, and the obtained sodium dichromate solution is adjusted in degree of acidification and then evaporated to precipitate crystals, so as to yield sodium dichromate dihydrate (see U.S. Pat. No. 5,250,274, 1993 for related techniques).
Among the above methods for preparation of dichromate, the sulfuric-acid method and the carbonation method produce Na2SO4 and NaHCO3 as side products, respectively, which result in a low economic effect; and the electrolysis method has high power consumption, and thus is yet to be improved.